The present invention relates to a safety switching device and a method for the failsafe shutdown of an electric load, in particular for shutting down an electrically driven machine. The invention particularly relates to a safety switching device and a method for the failsafe shutdown of an inductive load that causes overvoltage spikes when being shut down.
Safety switching devices in terms of the present invention are provided for controlling technical systems and devices in a failsafe manner, and in particular shutting them down in a failsafe manner, if required, in order to prevent, for example, a risk to operating personnel. “Failsafe” in this case means that the successful shutdown operation needs to be ensured even when a fault occurs in the safety switching device or in its environment, for example when a component of the safety switching device fails. Therefore, prior art safety switching devices are generally of a redundant design and, in numerous countries, they require special approval from a competent supervisory authority. With regard to the present invention, safety switching devices are those which at least meet Category 3 of the European Standard EN 954-1 or a comparable safety standard, and also those which are specifically envisaged for a corresponding safety-critical application. In this regard, however, the invention is not restricted to safety switching devices in the narrower sense, but it also comprises safety controllers and field bus-based safety systems, which implement the functionality of a safety switching device of the type to be described in more detail below in addition to possibly complex control tasks.
The signal processing part of the safety switching device serves the purpose of receiving and evaluating an input-side switching signal, which is produced in the case of conventional applications, for example, by an emergency stop button, a guard door, a light barrier arrangement, two-handed control switches or other safety transmitters. The signal processing part evaluates the switching signal in a failsafe manner and, on the basis thereof, controls at least one, but generally two or more, output-side switching elements. The output signal of the switching elements is fed to one or more actuators, such as contactors, via whose working contacts the current is supplied to a monitored machine. Furthermore, solenoid valves, which are part of a hydraulic drive, are often also switched on and off using known safety switching devices.
Contactors, solenoid valves and similar actuators have in common that they represent an inductive load for the safety switching device. This means that, during shutdown, an overvoltage spike occurs, whose level can substantially exceed the normal operating voltage. Therefore, the outputs of prior art safety switching devices generally have an anti-surge element, often what is called an anti-surge diode, which is connected back-to-back in parallel with the input coil of the contactor or solenoid valve. A safety switching device having such an anti-surge diode is known, for example, from DE 199 54 460 A1. In addition, safety switching devices having an anti-surge diode are marketed by the present assignee under the brand name PNOZ®.
However, the anti-surge elements in the known safety switching devices have two principal disadvantages which may be significant to differing extents depending on the practical implementation. A first disadvantage consists in the fact that the current through the contactor coil decays more slowly during shutdown, which delays the shutdown operation. A second disadvantage is undesirable introduction of heat into the safety switching device, which is produced, in particular, by the signal processing part needing to switch the output-side switching elements off and on again frequently for test purposes. The voltage spike which occurs during disconnection is canceled via the anti-surge element arranged in the safety switching device, and the energy stored in the contactor coil is converted into a resistive power loss within the safety switching device.